The present invention relates to the improvements in an ion micro beam apparatus which is used for ion micro analysis, ion micro doping, ion beam writing, and the like.
The ion micro beam apparatus irradiates the specimens with an ion beam which is finely focused into a beam diameter of about 1 .mu.m or smaller, and is expected to be adapted to a microfabrication process for producing semiconductor elements. The ion source consists of a point optical source having a high brightness, such as a liquid metal ion source, a field ionization ion source, a duoplasmatron, or the like. In particular, when several kinds of ions are obtained by employing, for example, the liquid metal ion source which turns an alloy into ionized substances, it is necessary to use a mass separator in order to select and separate ions of a desired kind.
FIG. 1 illustrates a conventional ion micro beam apparatus which chiefly consists of an ion source 2 having a high brightness, electrostatic lenses 3 and 4 which so focus an ion beam 15 emitted therefrom that an object 14 is projected onto images 13 and 13' along an optical axis 1, an electrostatic deflector 5 which deflects the ion beam on a specimen plate 6, and an E.times.B mass filter 7 which is a mass separator installed between the two electrostatic lenses 3 and 4. The E.times.B mass filter 7 consists of an E.times.B deflector 12 which is made up of electrodes 8a, 8b and a magnetic pole 9a to mass-separate the ion beam, and a mass separating aperture 10 to take out desired ions only. An aperture 11 restricts the ion beam current.
The mass separator in an ion micro beam apparatus should have a high mass resolution to remove ions of undesired kinds and, at the same time, should not adversely affect the beam focusing performance. The E.times.B mass filter 7 used as a mass separator enables the optical system to be linearly arranged, and makes it easy to carry out the design and axis alignment. However, the E.times.B mass filter 7 cannot sufficiently satisfy the above-mentioned two requirements simultaneously due to chromatic aberration. The reasons will be described below briefly.
An example which employs an E.times.B mass filter has been taught in Ishitani et al., "Mass-Separated Microbeam System with a Liquid-Metal-Ion Source", Nucl. Instr. and Meth. 218 (1983) 363.
First, the operation principle of the E.times.B mass filter will now be explained in conjunction with FIG. 2. The basic structural elements of the E.times.B mass filter include an E.times.B deflector 12 which consists of a pair of electrodes 8a, 8b and a pair of magnetic pole pieces 9a, 9b (9b is not diagramed) to generate an electric field E and a magnetic field B in the directions perpendicularly to the ion optical axis 1, respectively, and a mass separating aperture 10 of a subsequent stage. The ions incident along the ion optical axis 1 at an acceleration voltage V.sub.0 proceed straight when the masses thereof satisfy the following condition and pass the mass separating aperture 10, i.e., EQU (2eV.sub.0 /m).sup.1/2 =E/B (1)
On the other hand, ions of which the masses are different by .DELTA.m are deflected on the aperture 10 by .DELTA.Xm toward the direction of the electric field E from the optical axis 1, i.e., EQU .DELTA.Xm=(.DELTA.m/m)(E/V.sub.0)Lm(Ld+Lm/2)/4 (2)
Further, if the emitted ion energy spread of the ion source is .DELTA.V, ions of the mass m are also deflected by .DELTA.Xc.
This becomes important when the acceleration voltage is low or when the ion source consists of a liquid metal ion source having a large energy spread. EQU .DELTA.Xc=(.DELTA.V/V.sub.0)(E/V.sub.0)Lm(Ld+Lm/2)/4 (3)
From the above equation (2), the mass resolution (m/.DELTA.m) of the E.times.B mass filter can be defined as follows: EQU (m/.DELTA.m)=Lm/.gamma.A(E/V.sub.0)(Ld+Lm/2)/4 (4)
where .gamma.A denotes the width of the aperture 10.
Further, there exists an upper limit in the mass resolution since the beam has a width on the aperture 10 due to a finite aperture angle of the incident ion beam. That is, when .gamma.A.perspectiveto.d, in the equation (4), d denotes a beam width. Therefore, the mass resolution of the E.times.B mass filter is affected by the optical system of the preceding stage.
The above-mentioned problem will now be described with reference to FIG. 3 which illustrates the E.times.B mass filter used in the practical ion micro beam apparatus. FIGS. 3A and 3B illustrate the case where the incident ion beam 15 is focused on the central surface 21 of the E.times.B deflector 12 (FIG. 3B) by the lens of the preceding stage that is not shown, and the case where the incident ion beam 15 is focused on the mass separating aperture 10 (FIG. 3A).
In the case of FIG. 3B, the beam 16 is slightly spread as denoted by 18 due to .DELTA.Xc of the equation (3). However, the beam focusing performance is not adversely affected since the chromatic aberration at the focal point 13 is cancelled (this has been disclosed in detail, for example, in Japanese Patent Laid-Open No. 7550/1984). The mass resolution, however, is considerably small. In the case of FIG. 3A, .DELTA.Xc of the equation (3) serves as chromatic aberration at the focal points 13, 13' to greatly deteriorate the beam focusing performance. In particular, .DELTA.Xc does not change depending upon the aperture angle of the beam, and cannot be removed even if the aperture is inserted at the back of the lens 4. However, the beam is focused on the mass separating aperture 10, and the beam diameter is nearly equal to .DELTA.Xc. Therefore, the mass resolution is greater than that of the case of FIG. 3B.
Using the conventional E.times.B mass filter, as described above, it was not possible to remove the chromatic aberration and to fulfill high mass resolution as well as high beam focusing performance, simultaneously.
To cope with this problem, there has been proposed an example which employs a mass separator consisting of magnetic field filters of four stages (P. D. Prewett, Vacuum, 34, 931, 1984). This method can cancel the chromatic aberration, can focus the beam on the mass separating aperture, and offers the probability that high mass resolution can be obtained. However, since the ion trajectory is greatly deviated from the straight line due to the magnetic field, the astigmatism becomes great (the astigmatic correctors must be provided in the preceding and succeeding stages). Moreover, the mass resolution varies greatly depending not only upon the size of the mass separating aperture but also upon its position, and it becomes difficult to set the mass resolution. With this mass separator, furthermore, the magnetic field must be varied using electromagnets that are difficult to assemble in small sizes.